This invention relates to homopolar dynameolectric machines and more particularly to drum-type homopolar generators.
Homopolar generators have been successfully designed for providing short duration pulses having a peak current level in excess of a million amperes DC. Such generators generally include a cylindrical rotor of either a drum or disc configuration, mounted on a frame and rotated about a central axis. A field coil encircling the rotor and connected to an external current supply provides an applied magnetic field excitation passing through the rotor. The applied field excitation is usually confined and directed by a ferromagnetic stator structure surrounding the field coil and all, or a portion of, the rotor. When the rotor is spinning, free electrons within the rotor experience an electromotive force resulting from their interaction with the applied magnetic field excitation. Brushes, positioned adjacent to a current collection zone on the rotor, are then lowered onto the spinning rotor to allow an electrical current to flow under the influence of the electromotive force through return conductors to an external circuit, and then back onto the rotor through additional brushes at a different location. During the discharge, the interaction of the discharge current in the applied field excitation creates a force which decelerates the rotor. It has been found, that extremely high current pulses may be obtained by using a relatively low power conventional prime mover or a conventional low voltage, low amperage power source to store initial energy in the rotor by gradually motoring the rotor up to the desired rotational speed.
Drum-type homopolar dynamoelectric machines include a stationary excitation system and a rotating drum composed of a combination of ferromagnetic and highly conductive materials such that a direct current output voltage is produced along the axial length of the drum. These machines incorporate a set of current collecting brushes at axially displaced locations along the rotor surface, which carry full load current. Homopolar dynamoelectric machines may operate as either a motor or generator and are particularly suited to transfer energy in short, high current pulses to a storage inductor and a final load consisting of a resistive-inductive system. The rotor of drum-type homopolar machines may include a cylindrical shell of a highly conductive, non-ferromagnetic material which generates and supports the full load current. This member is bonded or shrunk onto a ferromagnetic inner cylindrical core which serves as the main rotor body and is directly attached to a drive or input shaft. Both components of the rotor are, preferably, homogeneous materials without segmentation or any combination of axial or circumferential grooves. Since modern current collectors may operate at a current density of between 10 and 15 kiloamps per square inch, it is imperative that the rotor surface near the two axial ends be smooth since this zone is used exclusively for current collection with, for example, solid metal graphite or fiber brushes. The machine's internal electromotive force is confined to an axial zone along the center of the rotor between the two outer current collection zones.
The rotor surface speed of a drum rotor in a high performance homopolar generator may exceed 100 meters per second and the generated current may exceed 1,000,000 amperes. Under those conditions, a large amount of heat is generated at the brush/rotor interface. The surface of the rotor directly under the brushes is heated by friction and electric power dissipated in the brush contact resistance. In some homopolar generator designs, the friction and brush contact losses will be in excess of 1 megawatt, resulting in high rotor surface temperatures. A third source of heating is the resistive losses resulting from current traveling axially through the conductive rotor shell. The amount of heat produced by each heat generating source is a function of the rotor length from the center plane of a typical homopolar generator to the peripheral edges of the brush assemblies. In prior art homopolar generators which include brush assemblies having a uniform packing factor, the region just at the beginning of each brush box has the highest resistive heating plus friction heating. The rotor conductor bulk temperature will therefore be the highest in this region. This results in high thermal stresses and increased brush wear. Furthermore, if the machine rotor includes a conductive shell which was heat shrunk on, this region might experience total relief of the heat shrunk fit. Therefore, it is desirable to design a homopolar dynameolectric machine in which the peak rotor temperatures in the vicinity of the brush assemblies are minimized.